-flto making program slower?

[Uri Moszkowicz]

Let me clarify then. The benchmark consisted of a few dozen runs of
the program each with different inputs, with the average time given.
For this program, each run typically takes a few hours so there's
little risk of sample error.

On Tue, Jun 11, 2013 at 6:01 PM, Hei Chan <structurechart@yahoo.com> wrote:
> I ran into similar issue but I got no response here.
>
> My guess is that flto makes your average run time smaller.  But I guess most
> people use it in a hope of making their critical pathes running faster,
> which are benchmarked.
>
>
> ________________________________
> From: Uri Moszkowicz <uri@4refs.com>
> To: gcc-help <gcc-help@gcc.gnu.org>
> Sent: Wednesday, June 12, 2013 5:48 AM
> Subject: -flto making program slower?
>
> Hi,
> I'm having trouble with link time optimization in my application. It
> is a large application that uses only basic C++ (no exceptions, no
> templates, no STL, no floating point, etc). Like many applications,
> the source files are compiled separately into object files. Some of
> those are combined into shared libraries. The shared libraries are
> then statically linked with the remaining object files to produce the
> final application, which is about 100MB big. We are using GCC 4.7.2
> with a non-GOLD 2.23.1 binutils.
>
> I simply added "-flto" to the GCC command to create object files:
>     g++ -Wall -pipe -O3 -flto -fno-strict-aliasing -mtune=generic
> --no-exceptions -fPIC -c some.cc
>
> I then added "-flto" to the final link command but not the shared libraries:
>   g++ -o exec -Xlinker some1.o some2.o -static some1.a some2.a
> -Wl,--wrap,open -flto
>
> I ran a benchmark of tests and the resulting execution time is now
> about 7% higher than it was without "-flto" added. Any suggestions for
> how to improve this result or why it may have gotten slower?
>
> If it helps, when I add "-fuse-linker-plugin" I get this error:
>   g++: error: -fuse-linker-plugin is not supported in this configuration
>
> Thanks!
>
>